News Article | November 18, 2016
An international team of astrophysicists including Russian scientists from the Space Research Institute of the Russian Academy of Sciences (RAS), MIPT, and Pulkovo Observatory of RAS has detected an abrupt decrease of pulsar luminosity following giant outbursts. The phenomenon is associated with the so-called "propeller effect," which was predicted more than 40 years ago. However, this is the first study to reliably observe the transition of the two X-ray pulsars 4U 0115+63 and V 0332+53 to the "propeller regime." The results of the observations, the conclusions reached by the researchers, and the relevant calculations were published in Astronomy & Astrophysics. The two sources studied, 4U 0115+63 and V 0332+53, belong to a rather special class of transient X-ray pulsars. These stars alternately act as weak X-ray sources, undergo giant outbursts, and disappear from sight completely. The transitions of pulsars between different states provide valuable information about their magnetic field and the temperature of the surrounding matter. Such information is indispensable, as the immensely strong magnetic fields and extremely high temperatures make direct measurements impossible in a laboratory on Earth. The name of a pulsar is preceded by a letter designating the first observatory to discover it, which is followed by a numerical code containing the coordinates of the pulsar. The "V" refers to Vela 5B, a US military satellite that was launched to spy on the Soviets. As for the "4U" in the other name, it stands for the fourth Uhuru catalog, compiled by the first observatory in orbit dedicated specifically to X-ray astronomy. Following the discovery of the first pulsar, it was originally known as "LGM-1" (for "little green men"), because it was a source of regular radio pulses, leading scientists to believe that they might have received a signal from intelligent extraterrestrials. An X-ray pulsar is a rapidly spinning neutron star with a strong magnetic field. A neutron star can be part of a binary system. In a process that astrophysicists call accretion, the neutron star can channel gas from its normal star companion. The attracted gas spirals toward the neutron star forming an accretion disk, which is disrupted at the magnetosphere radius. During accretion the matter penetrates to a certain extent into the magnetosphere, "freezes into it," and flows along the lines of the magnetic field toward the magnetic poles of the neutron star. Falling toward the poles, the gas is heated to several hundred million degrees, which causes the emission of X-rays. If the magnetic axis of a neutron star is skewed relative to its rotational axis, the X-ray beams it emits rotate in a manner that resembles the way beacons work. For an "onshore" observer, the source appears to be sending signals at regular intervals ranging from fractions of a second to several minutes. A neutron star is one of the possible remnants left behind by a supernova. It can be formed at the end of stellar evolution, if the original star was massive enough to allow gravitation to compress the stellar matter enough to make electrons combine with protons yielding neutrons. The magnetic field of a neutron star can be more than ten orders of magnitude stronger that any magnetic field that could be achieved on Earth. A binary system where the normal star has filled its Roche lobe. In a binary system, an X-ray pulsar is observed when the neutron star is accreting matter from its normal star companion--often a giant or a supergiant characterized by a strong stellar wind (ejection of matter into space). Alternatively, it can be a smaller star like our own Sun that has filled its Roche lobe--the region beyond which it is unable to hold on to the matter attracted by the gravity of the neutron star companion. A NASA video showing the accretion of matter by a pulsar from its companion star. The 4U 0115+63 and V 0332+53 pulsars are irregular X-ray sources (transients), owing to the fact that their stellar companions belong to the rather unusual Be star class. The axial rotation of a Be star is so rapid that it occasionally starts "bulging" at the equator, whereby a gas disk is formed around it, filling the Roche lobe. The neutron star starts rapidly accreting the gas from its "donor" companion, causing a sharp increase in X-ray emission called an X-ray outburst. At some point, after the matter in the equatorial bulge starts to deplete, the accretion disk becomes exhausted, and the gas can no longer fall onto the neutron star due to the influence of the magnetic field and the centrifugal force. This gives rise to a phenomenon known as the "propeller effect": the pulsar enters a state where accretion does not occur, and the X-ray source is no longer observed. Astronomers use the term "luminosity" to refer to the total amount of energy emitted by a celestial body per unit time. The red line in the diagram represents the threshold luminosity for the 4U 0115+63 pulsar. Observations of the other source (V 0332+53) produced similar results. The blue lines mark the moments in time, when the distance between the pulsar and the companion was at a minimum. This proximity of the companion star might cause the neutron star to go into overdrive and resume emission (see diagram), provided that sufficient amounts of matter are still available for accretion. The Russian scientists used the X-ray telescope (XRT) based on NASA's Swift space observatory to measure the threshold luminosity that marks the transition of a pulsar to the propeller regime. This parameter depends on the magnetic field and the rotational period of the pulsar. The rotational periods of the sources in this study are known based on the intervals between the pulses that we can register: 3.6 s in the case of 4U 0115+63 and 4.3 s for V 0332+53. Knowing both the threshold luminosity and the rotational period, one can calculate the strength of the magnetic field. The research findings are in agreement with the values obtained using other methods. However, the luminosity was only reduced by a factor of 200, as compared to the expected 400 times reduction. The researchers hypothesized that there could be two possible explanations for this discrepancy. Firstly, the neutron star surface could become an additional source of X-rays, as it cools down following an outburst. Secondly, the propeller effect could leave some room for matter transfer between the two stars, as opposed to sealing the neutron star off completely. In other words, an unaccounted for mechanism could be involved enabling accretion to continue to a certain extent. The transition of a pulsar into the propeller mode is challenging to observe, as the low luminosity state cannot be detected easily. For 4U 0115+63 and V 0332+53, this was attempted following the previous outbursts of these sources. However, the instruments available at the time were not sensitive enough to see the pulsars in the "off-mode." This study is the first to demonstrate reliably that these two sources do indeed "black out." Moreover, the researchers showed that knowledge of the luminosity that marks the transition of pulsars into the propeller regime can be used to learn more about the structure and intensity of the magnetic fields around neutron stars. Prof. Dr. Alexander Lutovinov of the Russian Academy of Sciences, Head of Laboratory at the Space Research Institute (IKI RAS) and a professor at MIPT, comments, "Knowledge of the structure of the magnetic fields of neutron stars is of paramount importance for our understanding of their formation and evolution. In this research, we determined the dipole magnetic field component, which is linked to the propeller effect, for two neutron stars. We demonstrate that this independently calculated value can be compared to the available results of magnetic field measurements based on the detection of cyclotron lines in the spectra of sources. By doing this, it is possible to estimate the contribution of the other, higher-order components in the field structure."
News Article | September 14, 2016
On its way to assembling the most detailed 3-D map ever made of our Milky Way galaxy, Gaia has pinned down the precise position on the sky and the brightness of 1142 million stars. As a taster of the richer catalogue to come in the near future, today's release also features the distances and the motions across the sky for more than two million stars. "Gaia is at the forefront of astrometry, charting the sky at precisions that have never been achieved before," says Alvaro Giménez, ESA's Director of Science. "Today's release gives us a first impression of the extraordinary data that await us and that will revolutionise our understanding of how stars are distributed and move across our Galaxy." Launched 1000 days ago, Gaia started its scientific work in July 2014. This first release is based on data collected during its first 14 months of scanning the sky, up to September 2015. "The beautiful map we are publishing today shows the density of stars measured by Gaia across the entire sky, and confirms that it collected superb data during its first year of operations," says Timo Prusti, Gaia project scientist at ESA. The stripes and other artefacts in the image reflect how Gaia scans the sky, and will gradually fade as more scans are made during the five-year mission. "The satellite is working well and we have demonstrated that it is possible to handle the analysis of a billion stars. Although the current data are preliminary, we wanted to make them available for the astronomical community to use as soon as possible," adds Dr Prusti. Transforming the raw information into useful and reliable stellar positions to a level of accuracy never possible before is an extremely complex procedure, entrusted to a pan-European collaboration of about 450 scientists and software engineers: the Gaia Data Processing and Analysis Consortium, or DPAC. "Today's release is the result of a painstaking collaborative work over the past decade," says Anthony Brown from Leiden University in the Netherlands, and consortium chair. "Together with experts from a variety of disciplines, we had to prepare ourselves even before the start of observations, then treated the data, packaged them into meaningful astronomical products, and validated their scientific content." In addition to processing the full billion-star catalogue, the scientists looked in detail at the roughly two million stars in common between Gaia's first year and the earlier Hipparcos and Tycho-2 Catalogues, both derived from ESA's Hipparcos mission, which charted the sky more than two decades ago. By combining Gaia data with information from these less precise catalogues, it was possible to start disentangling the effects of 'parallax' and 'proper motion' even from the first year of observations only. Parallax is a small motion in the apparent position of a star caused by Earth's yearly revolution around the Sun and depends on a star's distance from us, while proper motion is due to the physical movement of stars through the Galaxy. In this way, the scientists were able to estimate distances and motions for the two million stars spread across the sky in the combined Tycho–Gaia Astrometric Solution, or TGAS. This new catalogue is twice as precise and contains almost 20 times as many stars as the previous definitive reference for astrometry, the Hipparcos Catalogue. As part of their work in validating the catalogue, DPAC scientists have conducted a study of open stellar clusters – groups of relatively young stars that were born together – that clearly demonstrates the improvement enabled by the new data. "With Hipparcos, we could only analyse the 3-D structure and dynamics of stars in the Hyades, the nearest open cluster to the Sun, and measure distances for about 80 clusters up to 1600 light-years from us," says Antonella Vallenari from the Istituto Nazionale di Astrofisica (INAF) and the Astronomical Observatory of Padua, Italy. "But with Gaia's first data, it is now possible to measure the distances and motions of stars in about 400 clusters up to 4800 light-years away. For the closest 14 open clusters, the new data reveal many stars surprisingly far from the centre of the parent cluster, likely escaping to populate other regions of the Galaxy." Many more stellar clusters will be discovered and analysed in even greater detail with the extraordinary data that Gaia continues to collect and that will be released in the coming years. The new stellar census also contains 3194 variable stars, stars that rhythmically swell and shrink in size, leading to periodic brightness changes. Many of the variables seen by Gaia are in the Large Magellanic Cloud, one of our galactic neighbours, a region that was scanned repeatedly during the first month of observations, allowing accurate measurement of their changing brightness. Details about the brightness variations of these stars, 386 of which are new discoveries, are published as part of today's release, along with a first study to test the potential of the data. "Variable stars like Cepheids and RR Lyraes are valuable indicators of cosmic distances," explains Gisella Clementini from INAF and the Astronomical Observatory of Bologna, Italy. "While parallax is used to measure distances to large samples of stars in the Milky Way directly, variable stars provide an indirect, but crucial step on our 'cosmic distance ladder', allowing us to extend it to faraway galaxies." This is possible because some kinds of variable stars are special. For example, in the case of Cepheid stars, the brighter they are intrinsically, the slower their brightness variations. The same is true for RR Lyraes when observed in infrared light. The variability pattern is easy to measure and can be combined with the apparent brightness of a star to infer its true brightness. This is where Gaia steps in: in the future, scientists will be able to determine very accurate distances to a large sample of variable stars via Gaia's measurements of parallaxes. With those, they will calibrate and improve the relation between the period and brightness of these stars, and apply it to measure distances beyond our Galaxy. A preliminary application of data from the TGAS looks very promising. "This is only the beginning: we measured the distance to the Large Magellanic Cloud to test the quality of the data, and we got a sneak preview of the dramatic improvements that Gaia will soon bring to our understanding of cosmic distances," adds Dr Clementini. Knowing the positions and motions of stars in the sky to astonishing precision is a fundamental part of studying the properties and past history of the Milky Way and to measure distances to stars and galaxies, but also has a variety of applications closer to home – for example, in the Solar System. In July, Pluto passed in front of a distant, faint star, offering a rare chance to study the atmosphere of the dwarf planet as the star gradually disappeared and then reappeared behind Pluto. This stellar occultation was visible only from a narrow strip stretching across Europe, similar to the totality path that a solar eclipse lays down on our planet's surface. Precise knowledge of the star's position was crucial to point telescopes on Earth, so the exceptional early release of the Gaia position for this star, which was 10 times more precise than previously available, was instrumental to the successful monitoring of this rare event. Early results hint at a pause in the puzzling pressure rise of Pluto's tenuous atmosphere, something that has been recorded since 1988 in spite of the dwarf planet moving away from the Sun, which would suggest a drop in pressure due to cooling of the atmosphere. "These three examples demonstrate how Gaia's present and future data will revolutionise all areas of astronomy, allowing us to investigate our place in the Universe, from our local neighbourhood, the Solar System, to Galactic and even grander, cosmological scales," explains Dr Brown. This first data release shows that the mission is on track to achieve its ultimate goal: charting the positions, distances, and motions of one billion stars – about 1% of the Milky Way's stellar content – in three dimensions to unprecedented accuracy. "The road to today has not been without obstacles: Gaia encountered a number of technical challenges and it has taken an extensive collaborative effort to learn how to deal with them," says Fred Jansen, Gaia mission manager at ESA. "But now, 1000 days after launch and thanks to the great work of everyone involved, we are thrilled to present this first dataset and are looking forward to the next release, which will unleash Gaia's potential to explore our Galaxy as we've never seen it before." More information: The data from Gaia's first release can be accessed at archives.esac.esa.int/gaia Fifteen scientific papers describing the data contained in the release and their validation process will appear in a special issue of Astronomy & Astrophysics.
News Article | November 30, 2016
Not all galaxies sparkle with stars. Galaxies as wide as the Milky Way but bereft of starlight are scattered throughout our cosmic neighborhood. Unlike Andromeda and other well-known galaxies, these dark beasts have no grand spirals of stars and gas wrapped around a glowing core, nor are they radiant balls of densely packed stars. Instead, researchers find just a wisp of starlight from a tenuous blob. “If you took the Milky Way but threw away about 99 percent of the stars, that’s what you’d get,” says Roberto Abraham, an astrophysicist at the University of Toronto. How these dark galaxies form is unclear. They could be a whole new type of galaxy that challenges ideas about the birth of galaxies. Or they might be outliers of already familiar galaxies, black sheep shaped by their environment. Wherever they come from, dark galaxies appear to be ubiquitous. Once astronomers reported the first batch in early 2015 — which told them what to look for — they started picking out dark denizens in many nearby clusters of galaxies. “We’ve gone from none to suddenly over a thousand,” Abraham says. “It’s been remarkable.” This haul of ghostly galaxies is puzzling on many fronts. Any galaxy the size of the Milky Way should have no trouble creating lots of stars. But it’s still unclear how heavy the dark galaxies are. Perhaps these shadowy entities are failed galaxies, as massive as our own but mysteriously prevented from giving birth to a vast stellar family. Or despite being as wide as the Milky Way, they could be relative lightweights stretched thin by internal or external forces. Either way, with so few stars, dark galaxies must have enormous deposits of unseen matter to resist being pulled apart by the gravity of other galaxies. Astronomers can’t resist a good cosmic mystery. With detections of these galactic oddballs piling up, there is a push to figure out just how many of these things are out there and where they’re hiding. “There are more questions than answers,” says Remco van der Burg, an astrophysicist at CEA Saclay in France. Cracking the code of dark galaxies could provide insight into how all galaxies, including the Milky Way, form and evolve. Telescopes designed to detect faint objects have revealed the presence of many sizable but near-empty galaxies — officially known as “ultradiffuse galaxies.” The deluge of discoveries started in New Mexico, with a telescope that looks more like a honeycomb than a traditional observatory. Sitting in a park about 110 kilometers southwest of Roswell (a city that has turned extraterrestrials into a tourism industry), the Dragonfly telescope consists of 48 telephoto lenses; it started with three in 2013 and continues to grow. The lenses are divided evenly among two steerable racks, and each lens is hooked up to its own camera. Partly inspired by the compound eye found in dragonflies and other insects, this relatively small scope has revealed dim galaxies missed by other observatories. The general rule for telescopes is that bigger is better. A large mirror or lens can collect more light and therefore see fainter objects. But even the biggest telescopes have a limitation: unwanted light. Every surface in a telescope is an opportunity for light coming in from any direction to reflect onto the image. The scattered light shows up as dim blobs, or “ghosts,” that can wash out faint detail in pictures of space or even mimic very faint galaxies. Large dark galaxies look a lot like these ghosts, and so went unnoticed. But Dragonfly was designed to keep these splashes of light in check. Unlike most conventional professional telescopes, it has no mirrors. Precision antireflection coatings on the lenses keep scattered light to a minimum. And having multiple cameras pointed at the same part of the sky helps distinguish blobs of light bouncing around in the telescope from blobs that actually sit in deep space. If the same blob shows up in every camera, it’s probably real. “It’s a very clever idea, very brilliant,” says astronomer Jin Koda of Stony Brook University in New York. “Dragonfly made us realize that there is a chance to find a new population of galaxies beyond the boundary of what we know so far.” In spring 2014, researchers pointed Dragonfly at the well-studied Coma cluster, a conglomeration of thousands of galaxies. At a distance of about 340 million light-years, Coma is a close, densely packed collection of galaxies and a rich hunting ground for astronomers. A team led by Abraham and astronomer Pieter van Dokkum of Yale University was looking at the edges of galaxies for far-flung stars and stellar streams, evidence of the carnage left behind after small galaxies collided to build larger ones. They were not expecting to find dozens of galaxies hiding in plain sight. “People have been studying Coma for 80 years,” Abraham says. “How could we find anything new there?” And yet, scattered throughout the cluster appeared 47 dark galaxies, many of them comparable in size to the Milky Way — tens of thousands to hundreds of thousands of light-years across (SN: 12/13/14, p. 9). This was perplexing. A galaxy that big should have no problem forming lots of stars, van Dokkum and colleagues noted in September in Astrophysical Journal Letters. Even more surprising, says Abraham, is that those galaxies survive in Coma, a cluster crowded with galactic bullies. A galaxy’s own gravity holds it together, but gravity from neighboring galaxies can pull hard enough to tear apart a smaller one. To create sufficient gravity to survive, a galaxy needs mass in the form of stars, gas and other cosmic matter. In a place like Coma, a galaxy needs to be fairly massive or compact. But with so few stars (and presumably so little mass) spread over a relatively large space, dark galaxies should have been shredded long ago. They are either recent arrivals to Coma or a lot stronger than they appear. From what researchers have learned so far, dark galaxies seem to have been lurking for many billions of years. They are located throughout their home clusters, suggesting that they’ve had a long time to spread out among the other galaxies. And the meager stars they have are mostly red, indicating that they are very old. With this kind of longterm survival, dark galaxies probably have a hidden strength, most likely due to dark matter. All galaxies are loaded with dark matter, a mysterious substance that reveals itself only via gravitational interactions with luminous gas and stars. Much of that dark matter sits in an extended blob (known as the halo) that reaches well beyond the visible edge of a galaxy. On average, dark matter accounts for about 85 percent of all the matter in the universe. Within the central regions of the dark galaxies in Coma, dark matter must make up about 98 percent of the mass for there to be enough gravity to keep the galaxy intact, van Dokkum and colleagues say. Dark galaxies appear to have similar fractions of dark matter focused near their cores as the Milky Way does throughout its broader halo. Astronomers had never seen such a strong preference for dark matter in galaxies so large. The initial cache of galactic enigmas lured a slew of researchers to the hunt. They pored over existing images of Coma and other clusters, looking for more dark galaxies. These galaxies are so faint that they could easily blend in with a cluster’s background light or be mistaken for reflections within a telescope. But once the galaxy hunters knew what to look for, they were not disappointed — those first 47 were just the tip of the iceberg. Looking at old images of Coma taken by the Subaru telescope in Hawaii, Koda and colleagues easily confirmed that those 47 were really there. But that wasn’t all. They found a total of 854 dark galaxies, 332 of which appeared to be roughly the size of the Milky Way (SN: 7/25/15, p. 11). They calculated that Coma could harbor more than 1,000 dark galaxies of all sizes — comparable to its number of known galaxies. Astronomer Christopher Mihos of Case Western Reserve University in Cleveland and colleagues, reporting in 2015 in Astrophysical Journal Letters, found three more in the Virgo cluster, a more sparsely populated but closer gathering of galaxies that’s a mere 54 million light-years away. In June, van der Burg and collaborators reported another windfall in Astronomy & Astrophysics. Using the Canada-France-Hawaii Telescope atop Mauna Kea in Hawaii, they measured the masses of several galaxy clusters. Taking a closer look at eight clusters, all less than about 1 billion light-years away, the group found roughly 800 more ultradiffuse galaxies. “As we go to bigger telescopes, we find more and more,” says Michael Beasley, an astrophysicist at Instituto de Astrofísica de Canarias in Santa Cruz de Tenerife, Spain. “We don’t know how many there are, but we know there are a lot of them.” There could even be more dark galaxies than bright ones. What dark galaxies are and how they formed is still a mystery. There are many proposals, but with so little data, few conclusions. For the vast majority of dark galaxies, researchers know only how big and how bright each one is. Three so far have had their masses measured. Of those, two appear to have more in common masswise with some of the small galaxies that orbit the Milky Way, while the third is as massive as our galaxy itself — roughly 1 trillion times as massive as the sun. A dark galaxy in the Virgo cluster, VCC 1287, and another in Coma, Dragonfly 17, each have a total mass of about 70 billion to 90 billion suns. But only about one one-thousandth of that or less is in stars. The rest is dark matter. That puts the total masses of these two galaxies on par with the Large Magellanic Cloud, the largest of the satellite galaxies that orbit the Milky Way. But focus on just the mass of the stars, and the Large Magellanic Cloud is about 35 times as large as Dragonfly 17 and roughly 100 times as large as VCC 1287. A galaxy dubbed Dragonfly 44, however, is another story. It’s a dark beast, weighing about as much as the entire Milky Way and made almost entirely of dark matter, van Dokkum and colleagues report in September in Astrophysical Journal Letters. “It’s a bit of a puzzle,” Beasley says. “If you look at simulations of galaxy formation, you expect to have many more stars.” For some reason, this galaxy came up short. The environment may be to blame. A cluster like Coma grows over time by drawing in galaxies from the space around it. As galaxies fall into the cluster, they feel a headwind as they plow through the hot ionized gas that permeates the cluster. The headwind can strip gas from an incoming galaxy. But galaxies need gas to form stars, which are created when self-gravity crushes a blob of dust and gas until it turns into a thermonuclear furnace. If a galaxy falls into the cluster just as it is starting to make stars, this headwind might remove enough gas to prevent many stars from forming, leaving the galaxy sparsely populated. Or maybe there’s something intrinsic to a galaxy that turns it dark. A volley of supernovas or a prolific burst of star formation might drive gas out of the galaxy. Nicola Amorisco of the Max Planck Institute for Astrophysics in Garching, Germany, and Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., suggest that ultradiffuse galaxies start off as small galaxies that spun rapidly as they formed. All galaxies rotate, but perhaps dark galaxies are a subset that twirl so fast that their stars and gas have spread out, turning them into diffuse blobs rather than star-building machines. To test these and other ideas, astronomers are focused on two key pieces of information: the masses of these galaxies and their locations in the universe. Mass can help researchers distinguish between formation scenarios, such as whether or not dark galaxies are failed Milky Way–like behemoths. A survey of other locales would indicate whether dark galaxies are unique to big clusters such as Coma, suggesting that the environment plays a role in their creation. But if they turn up outside of clusters, isolated or with small groups of galaxies, then perhaps they’re just born that way. There’s already a hint that dark galaxies depend more on nature than nurture. Yale astronomer Allison Merritt and colleagues reported in October online at arXiv.org that four ultradiffuse galaxies lurk in a small galactic gathering about 88 million light-years away, indicating that clusters aren’t the only place dark galaxies can be found. And van der Burg, in his survey of eight clusters, found that dark galaxies make up the same fraction of all galaxies in a cluster regardless of cluster mass — at least, for clusters weighing between 100 trillion and 1 quadrillion times the mass of the sun. About 0.2 percent of the mass of the stars is tied up in the dark galaxies. Since all eight clusters host roughly the same relative number of dark galaxies, that suggests that there is something intrinsic about a galaxy that makes it dark, van der Burg says. What this all means for understanding how galaxies form is hard to say. These cosmic specters might be an entirely new entity that will require new ideas about galaxy formation. Or they could be one page from the galaxy recipe book. Timing, location and luck might send some of our heavenly neighbors toward a bright future and force others to fade into the background. Perhaps dark galaxies are a mixed bag, the end result of many different processes going on in a variety of environments. “I see no reason why the universe couldn’t make these things in many ways,” Abraham says. “Part of the fun over the next few years will be to figure out which is in play in any particular galaxy and what sort of objects the universe has chosen to make.” What is clear is that as astronomers push to new limits — fainter, farther, smaller — the universe turns up endless surprises. Even in Coma, a locale that has been intensively studied for decades, there are still things to discover. “There’s just a ton of stuff out there that we’re going to find,” Abraham says. “But what that is, I don’t know.” This story appears in the December 10, 2016, issue of Science News with the headline, "Dark Galaxies: Astronomers detect a plethora of star-starved giants."
News Article | November 10, 2016
Astronomers may have solved the mystery of the peculiar volatile behavior of a supermassive black hole at the center of a galaxy. Combined data from NASA's Chandra X-ray Observatory and other observatories suggest that the black hole is no longer being fed enough fuel to make its surroundings shine brightly. Many galaxies have an extremely bright core, or nucleus, powered by material falling toward a supermassive black hole. These so-called "active galactic nuclei" or AGN, are some of the brightest objects in the Universe. Astronomers classify AGN into two main types based on the properties of the light they emit. One type of AGN tends to be brighter than the other. The brightness is generally thought to depend on either or both of two factors: the AGN could be obscured by surrounding gas and dust, or it could be intrinsically dim because the rate of feeding of the supermassive black hole is low. Some AGN have been observed to change once between these two types over the course of only 10 years, a blink of an eye in astronomical terms. However, the AGN associated with the galaxy Markarian 1018 stands out by changing type twice, from a faint to a bright AGN in the 1980s and then changing back to a faint AGN within the last five years. A handful of AGN have been observed to make this full-cycle change, but never before has one been studied in such detail. During the second change in type the Markarian 1018 AGN became eight times fainter in X-rays between 2010 and 2016. After discovering the AGN's fickle nature during a survey project using ESO's Very Large Telescope (VLT), astronomers requested and received time to observe it with both NASA's Chandra X-ray Observatory and Hubble Space Telescope. The accompanying graphic shows the AGN in optical light from the VLT (left) with a Chandra image of the galaxy's central region in X-rays showing the point source for the AGN (right). Data from ground-based telescopes including the VLT allowed the researchers to rule out a scenario in which the increase in the brightness of the AGN was caused by the black hole disrupting and consuming a single star. The VLT data also cast doubt on the possibility of obscuration by intervening gas causing the dimming. However, the true mechanism responsible for the AGN's surprising variation remained a mystery until Chandra and Hubble data was analyzed. Chandra observations in 2010 and 2016 conclusively showed that obscuration by intervening gas was not responsible for the decline in brightness. Instead, models of the optical and ultraviolet light detected by Hubble, NASA's Galaxy Evolution Explorer and the Sloan Digital Sky Survey in the bright and faint states showed that the AGN had faded because the black hole was being starved of infalling material. This starvation also explains the fading of the AGN in X-rays. One possible explanation for this starvation is that the inflow of fuel is being disrupted. This disruption could be caused by interactions with a second supermassive black hole in the system. A black hole binary is possible as the galaxy is the product of a collision and merger between two large galaxies, each of which likely contained a supermassive black hole in its center. The list observatories used in this finding also include NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) mission and Swift spacecraft. Two papers, one with the first author of Bernd Husemann (previously at ESO and currently at the Max Planck Institute for Astronomy) and the other with Rebecca McElroy (University of Sydney), describing these results appeared in the September 2016 issue of Astronomy & Astrophysics journal. NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations. Please follow SpaceRef on Twitter and Like us on Facebook.
News Article | November 10, 2016
Many galaxies have an extremely bright core, or nucleus, powered by material falling toward a supermassive black hole. These so-called "active galactic nuclei" or AGN, are some of the brightest objects in the Universe. Astronomers classify AGN into two main types based on the properties of the light they emit. One type of AGN tends to be brighter than the other. The brightness is generally thought to depend on either or both of two factors: the AGN could be obscured by surrounding gas and dust, or it could be intrinsically dim because the rate of feeding of the supermassive black hole is low. Some AGN have been observed to change once between these two types over the course of only 10 years, a blink of an eye in astronomical terms. However, the AGN associated with the galaxy Markarian 1018 stands out by changing type twice, from a faint to a bright AGN in the 1980s and then changing back to a faint AGN within the last five years. A handful of AGN have been observed to make this full-cycle change, but never before has one been studied in such detail. During the second change in type the Markarian 1018 AGN became eight times fainter in X-rays between 2010 and 2016. After discovering the AGN's fickle nature during a survey project using ESO's Very Large Telescope (VLT), astronomers requested and received time to observe it with both NASA's Chandra X-ray Observatory and Hubble Space Telescope. The accompanying graphic shows the AGN in optical light from the VLT (left) with a Chandra image of the galaxy's central region in X-rays showing the point source for the AGN (right). Data from ground-based telescopes including the VLT allowed the researchers to rule out a scenario in which the increase in the brightness of the AGN was caused by the black hole disrupting and consuming a single star. The VLT data also cast doubt on the possibility that changes in obscuration by intervening gas cause changes in the brightness of the AGN. However, the true mechanism responsible for the AGN's surprising variation remained a mystery until Chandra and Hubble data was analyzed. Chandra observations in 2010 and 2016 conclusively showed that obscuration by intervening gas was not responsible for the decline in brightness. Instead, models of the optical and ultraviolet light detected by Hubble, NASA's Galaxy Evolution Explorer (GALEX) and the Sloan Digital Sky Survey in the bright and faint states showed that the AGN had faded because the black hole was being starved of infalling material. This starvation also explains the fading of the AGN in X-rays. One possible explanation for this starvation is that the inflow of fuel is being disrupted. This disruption could be caused by interactions with a second supermassive black hole in the system. A black hole binary is possible as the galaxy is the product of a collision and merger between two large galaxies, each of which likely contained a supermassive black hole in its center. The list observatories used in this finding also include NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) mission and Swift spacecraft. Two papers, one with the first author of Bernd Husemann (previously at ESO and currently at the Max Planck Institute for Astronomy) and the other with Rebecca McElroy (University of Sydney), describing these results appeared in the September 2016 issue of Astronomy & Astrophysics journal. Explore further: Starving black hole returns brilliant galaxy to the shadows More information: R. E. McElroy et al. The Close AGN Reference Survey (CARS), Astronomy & Astrophysics (2016). DOI: 10.1051/0004-6361/201629102 , https://arxiv.org/abs/1609.04423
News Article | November 4, 2016
Never before have astrophysicists measured light of such high energy from a celestial object so far away. Around 7 billion years ago, a huge explosion occurred at the black hole in the center of a galaxy. This was followed by a burst of high-intensity gamma rays. A number of telescopes, MAGIC included, have succeeded in capturing this light. An added bonus: it was thus possible to reconfirm Einstein's General Theory of Relativity, as the light rays encountered a less distant galaxy en route to Earth - and were deflected by this so-called gravitational lens. The object QSO B0218+357 is a blazar, a specific type of black hole. Researchers now assume that there is a supermassive black hole at the center of every galaxy. Black holes, into which matter is currently plunging are called active black holes. They emit extremely bright jets. If these bursts point towards Earth, the term blazar is used. The event now described in "Astronomy & Astrophysics" took place 7 billion years ago, when the universe was not even half its present age. "The blazar was discovered initially on 14 July 2014 by the Large Area Telescope (LAT) of the Fermi satellite," explains Razmik Mirzoyan, scientist at the Max Planck Institute for Physics and spokesperson for the MAGIC collaboration. "The gamma ray telescopes on Earth immediately fixed their sights on the blazer in order to learn more about this object." One of these telescopes was MAGIC, on the Canary Island of La Palma, specialized in high-energy gamma rays. It can capture photons - light particles - whose energy is 100 billion times higher than the photons emitted by our Sun and a thousand times higher than those measured by Fermi-LAT. The MAGIC scientists were initially out of luck, however: A full moon meant the telescope was not able to operate during the time in question. Eleven days later, MAGIC got a second chance, as the gamma rays emitted by QSO B0218+357 did not take the direct route to Earth: One billion years after setting off on their journey, they reached the galaxy B0218+357G. This is where Einstein's General Theory of Relativity came into play. This states that a large mass in the universe, a galaxy, for example, deflects light of an object behind it. In addition, the light is focused as if by a gigantic optical lens - to a distant observer, the object appears to be much brighter, but also distorted. The light beams also need different lengths of time to pass through the lens, depending on the angle of observation. This gravitational lens was the reason that MAGIC was able, after all, to measure QSO B0218+357 - and thus the most distant object in the high-energy gamma ray spectrum. "We knew from observations undertaken by the Fermi space telescope and radio telescopes in 2012 that the photons that took the longer route would arrive 11 days later," says Julian Sitarek (University of ?ódz, Poland), who led this study. "This was the first time we were able to observe that high-energy photons were deflected by a gravitational lens." Doubling the size of the gamma-ray universe The fact that gamma rays of such high energy from a distant celestial body reach Earth's atmosphere is anything but obvious. "Many gamma rays are lost when they interact with photons which originate from galaxies or stars and have a lower energy," says Mirzoyan. "With the MAGIC observation, the part of the universe that we can observe via gamma rays has doubled." The fact that the light arrived on Earth at the time calculated could rattle a few theories on the structure of the vacuum - further investigations, however, are required to confirm this. "The observation currently points to new possibilities for high-energy gamma ray observatories - and provides a pointer for the next generation of telescopes in the CTA project," says Mirzoyan, summing up the situation.
News Article | February 15, 2017
The open cluster M17 (the Omega Nebula), about 5000 light years away, is one of the brightest star-forming regions in the Milky Way. This infrared image from the 2MASS catalog reveals the ten surveyed young, massive stars that lie hidden in the gas and dust in this stellar nursery. Credit: Maria Ramirez-Tannus/UvA Astronomers from Leuven (Belgium) and Amsterdam (Netherlands) have discovered that massive stars in the star-forming region M17 (the Omega Nebula) are—against expectations—not part of a close binary. They have started their lives alone or with a distant partner star. The researchers base their findings on data from the X-shooter spectrograph on ESO's Very Large Telescope in northern Chile. The study will be published in Astronomy & Astrophysics Letters. The Omega Nebula is an open cluster in the constellation Sagittarius. The cluster is at a distance of about 5,000 light years and contains some dozens of young, hot stars. Hugues Sana (University of Leuven), Maria Ramirez-Tannus, Lex Kaper and Alex de Koter (University of Amsterdam) discovered that these massive stars have surprisingly little differences in their radial velocity, the speed towards us or away from us. If these stars were binaries their radial velocity would likely differ by tens to hundreds of kilometers per second because they are in their orbits around each other. In M17 it ranges with only five kilometers per second. Most stars are not alone. Recent research shows that 70 percent of the massive stars (some 10 to 100 times the mass of the Sun), which end their lives as neutron star or black hole, has one or more near companions. As a contrast, a statistical analysis shows that only about 10 percent of the massive stars in M17 are narrow binaries. Alternatively, M17 may contain a lot of wide binaries, compared with older star forming regions harboring both narrow and wide binaries. This is the first time that such a young star-forming region is examined for the presence of binary stars. The reason is that such areas are hidden from view by the gas and dust from which the new stars are formed. It is therefore a challenge to get spectra of high quality, from which the radial velocity can be determined. First author Sana: "If M17 has indeed no narrow binaries, these systems have to appear later in evolution. Maybe they are only wide binaries, which later migrate towards each other. Co-author Ramirez-Tannus is enthusiastic about the results: "We have now observed ten of them and will study many more to understand how wide binaries change in narrow binary stars." The answer to the question whether massive stars usually live together in binaries is important for understanding of the star formation process. However, it is also an indication for the formation of the number of neutron binaries and double black holes, which eventually can produce a gravitational wave. Explore further: Simple view of gravity does not fully explain distribution of stars in crowded clusters More information: A dearth of short-period massive binaries in the young massive star forming region M17: Evidence for a large orbital separation at birth? arxiv.org/abs/1702.02153
News Article | February 24, 2017
A three-color composite of the mid-infrared images of Saturn on Jan. 23, 2008 captured with COMICS on the Subaru Telescope. The Cassini Division and the C ring appear bright. Color differences reflect the temperatures; the warmer part is blue, the cooler part is red. Credit: NAOJ A team of researchers has succeeded in measuring the brightnesses and temperatures of Saturn's rings using the mid-infrared images taken by the Subaru Telescope in 2008. The images are the highest resolution ground-based views ever made. They reveal that, at that time, the Cassini Division and the C ring were brighter than the other rings in the mid-infrared light and that the brightness contrast appeared to be the inverse of that seen in the visible light (Figure 1). The data give important insights into the nature of Saturn's rings. The beautiful appearance of Saturn and its rings has always fascinated people. The rings consist of countless numbers of ice particles orbiting above Saturn's equator. However, their detailed origin and nature remain unknown. Spacecraft- and ground-based telescopes have tackled that mystery with many observations at various wavelengths and methods. The international Cassini mission led by NASA has been observing Saturn and its rings for more than 10 years, and has released a huge number of beautiful images. The Subaru Telescope also has observed Saturn several times over the years. Dr. Hideaki Fujiwara, Subaru Public Information Officer/Scientist, analyzed data taken in January 2008 using the Cooled Mid-Infrared Camera and Spectrometer (COMICS) on the telescope to produce a beautiful image of Saturn for public information purposes. During the analysis, he noticed that the appearance of Saturn's rings in the mid-infrared part of the spectrum was totally different from what is seen in the visible light Saturn's main rings consist of the C, B, and A rings, each with different populations of particles. The Cassini Division separates the B and A rings. The 2008 image shows that the Cassini Division and the C ring are brighter in the mid-infrared wavelengths than the B and A rings appear to be (Figure 1). This brightness contrast is the inverse of how they appear in the visible light, where the B and A rings are always brighter than the Cassini Division and the C ring (Figure 2). "Thermal emission" from ring particles is observed in the mid-infrared, where warmer particles are brighter. The team measured the temperatures of the rings from the images, which revealed that the Cassini Division and the C ring are warmer than the B and A rings. The team concluded that this was because the particles in the Cassini Division and C ring are more easily heated by solar light due to their sparser populations and darker surfaces. On the other hand, in the visible light, observers see sunlight being reflected by the ring particles. Therefore, the B and A rings, with their dense populations of particles, always seem bright in the visible wavelengths, while the Cassini Division and the C ring appear faint. The difference in the emission process explains the inverse brightnesses of Saturn's rings between the mid-infrared and the visible-light views. It turns out that the Cassini Division and the C ring are not always brighter than the B and A rings, even in the mid-infrared. The team investigated images of Saturn's rings taken in April 2005 with COMICS, and found that the Cassini Division and the C ring were fainter than the B and A rings at that time, which is the same contrast to what was seen in the visible light (Figure 3). The team concluded that the "inversion" of the brightness of Saturn's rings between 2005 and 2008 was caused by the seasonal change in the ring opening angle to the Sun and Earth. Since the rotation axis of Saturn inclines compared to its orbital plane around the Sun, the ring opening angle to the Sun changes over a 15-year cycle. This makes a seasonal variation in the solar heating of the ring particles. The change in the opening angle viewed from the Earth affects the apparent filling factor of the particles in the rings. These two variations - the temperature and the observed filling factor of the particles - led to the change in the mid-infrared appearance of Saturn's rings. The data taken with the Subaru Telescope revealed that the Cassini Division and the C ring are sometimes bright in the mid-infrared though they are always faint in visible light. "I am so happy that the public information activities of the Subaru Telescope, of which I am in charge, led to this scientific finding," said Dr. Fujiwara. "We are going to observe Saturn again in May 2017 and hope to investigate the nature of Saturn's rings further by taking advantages of observations with space missions and ground-based telescopes." This research is published in Astronomy & Astrophysics, Volume 599, A29 and posted on-line on February 23, 2017 (Fujiwara et al., 2017, "Seasonal variation of the radial brightness contrast of Saturn's rings viewed in mid-infrared by Subaru/COMICS"). More information: Hideaki Fujiwara et al, Seasonal variation of the radial brightness contrast of Saturn's rings viewed in mid-infrared by Subaru/COMICS, Astronomy & Astrophysics (2017). DOI: 10.1051/0004-6361/201527529
News Article | February 15, 2017
In turn, they have used that result to calculate that the Sun is approximately 7.9 kiloparsecs from the Galaxy's centre—or almost twenty-six thousand light-years. Using data from the Gaia space telescope and the RAdial Velocity Experiment (RAVE) survey, Jason Hunt and his colleagues determined the velocities of over 200,000 stars relative to the Sun. Hunt is a Dunlap Fellow at the Dunlap Institute for Astronomy & Astrophysics, University of Toronto. The collaborators found an unsurprising distribution of relative velocities: there were stars moving slower, faster and at the same rate as the Sun. But they also found a shortage of stars with a Galactic orbital velocity of approximately 240 kilometres per second slower than the Sun's. The astronomers concluded that the missing stars had been stars with zero angular momentum; i.e. they had not been circling the Galaxy like the Sun and the other stars in the Milky Way Galaxy; "Stars with very close to zero angular momentum would have plunged towards the Galactic centre where they would be strongly affected by the extreme gravitational forces present there," says Hunt. "This would scatter them into chaotic orbits taking them far above the Galactic plane and away from the Solar neighbourhood." "By measuring the velocity with which nearby stars rotate around our Galaxy with respect to the Sun," says Hunt, "we can observe a lack of stars with a specific negative relative velocity. And because we know this dip corresponds to 0 km/sec, it tells us, in turn, how fast we are moving." Hunt and his colleagues then combined this finding with the proper motion of the supermassive blackhole known as Sagittarius A* ("A-star") that lies at the centre of the Galaxy to calculate the 7.9 kiloparsec distance. Proper motion is the motion of an object across the sky relative to distant background objects. They calculated the distance in the same way a cartographer triangulates the distance to a terrestrial landmark by observing it from two different positions a known distance apart. The result was published in Astrophysical Journal Letters in December 2017. The method was first used by Hunt's co-author, current chair of the Department of Astronomy & Astrophysics at the University of Toronto, Prof. Ray Calberg, and Carlberg's collaborator, Prof. Kimmo Innanen. But the result Carlberg and Innanen arrived at was based on less than 400 stars. Gaia is creating a dynamic, three-dimensional map of the Milky Way Galaxy by measuring the distances, positions and proper motion of stars. Hunt and his colleagues based their work on the initial data release from Gaia which included hundreds of thousands of stars. By the end of its 5 year mission, the space mission will have mapped well over 1 billion stars. The velocity and distance results are not significantly more accurate than other measurements. But according to Hunt, "Gaia's final release in late 2017 should enable us to increase the precision of our measurement of the Sun's velocity to within approximately one km/sec, which in turn will significantly increase the accuracy of our measurement of our distance from the Galactic centre." Explore further: Astronomers detect a fast rotating group of stars in our galaxy More information: Jason A. S. Hunt et al. DETECTION OF A DEARTH OF STARS WITH ZERO ANGULAR MOMENTUM IN THE SOLAR NEIGHBORHOOD, The Astrophysical Journal (2016). DOI: 10.3847/2041-8205/832/2/L25
News Article | November 10, 2016
Based on computer simulations, Astrophysicists at the University of Bern, Switzerland, conclude that the comet Chury did not obtain its duck-like form during the formation of our solar system 4.5 billion years ago. Although it does contain primordial material, they are able to show that the comet in its present form is hardly more than a billion years old. Based on data from the Rosetta space probe, scientists have so far assumed that the comet 67P/Churyumov-Gerasimenko originated from the initial phase of our solar system. Its peculiar, duck-shaped structure would have resulted from a gentle collision of two objects about 4.5 billion years ago. Based on new research, Martin Jutzi and Willy Benz from NCCR PlanetS and the Center for Space and Habitability (CSH) of the University of Bern, together with colleagues, have now come to a different conclusion. As a result of two studies published in the journal Astronomy & Astrophysics, Astrophysicist Martin Jutzi explains that "It is unlikely that a body like Chury has survived for such a long time without damage -- our computer simulations show this. " If the assumptions of the present "standard" model of the origin of our solar system are correct, a quiet initial phase was followed by a period in which large bodies initiated higher velocities and more violent collisions. In a first study, the scientists calculated how much energy would be needed to destroy a structure like Chury in a collision. As it turned out, Chury has a weak point; the connection between the two parts -- the neck between the head and the body. "We have found that this structure can be destroyed easily, even with low energy collisions," Martin Jutzi summarizes. Willy Benz compares the neck of the comet with the stem of a glass: "A dishwasher has to clean very gently, so that the stem of the glass does not break," says the astrophysicist. Obviously, the solar system did not handle this aspect as carefully. The new study shows that comets like Chury experienced a significant number of collisions over time, the energy of which would have been sufficient to destroy a bi-lobe structure. Therefore, the shape is not primordial, but has developed through collisions over billions of years. "Chury's present shape is the result of the last major impact which probably occurred within the last billion years," says Martin Jutzi. The duck-shaped Chury is therefore much younger than previously thought. The only alternative would be that the current standard model of the early evolution of the Solar System is not correct and there were fewer small objects than previously thought. In this case there would not have been as many collisions and Chury would have had the chance to keep its primordial shape. "At the moment, we do think though that Chury's shape is the result of many collisions, and that the standard model doesn't need to be revised," says Jutzi. In the second paper, Jutzi and Benz investigate exactly how Chury's current form could have resulted from a collision. In their computer models, they had small objects with a diameter of 200 to 400 meters crashing into a roughly five-kilometre, rotating body in the form of a rugby ball (see animation). The impact speed was in the range of 200 to 300 meters per second, which clearly exceeds the escape velocity for objects of this size (about 1 meter per second). However, the energy involved is still far below that of a catastrophic impact in which a large part of the body is pulverized. As a result, the target was torn in two parts, which, due to the effects of their mutual gravitational force, later merged into a structure with two parts -- a structure like Chury. Does the result of this research contradict previous knowledge that comets consist of primordial material at least as old as our solar system? "No," the researchers say. Their computer simulations show that the relatively small impact energy does not heat or compress the comet globally. The body is still porous and the volatile material which was contained in it since the beginning is retained. In connection with Chury, these properties could be measured convincingly with the space probe Rosetta. "So far, it has been assumed that comets are original building blocks -- similar to Lego," says Willy Benz. "Our work shows that the Lego blocks no longer have their original form, but the plastic that they consist of is still the same as in the beginning."